1. Field of the Invention
The present invention relates to an absorber pipe, in particular for solar collectors in solar thermal power plants, having at least one collector mirror, comprising a metal pipe for conducting and heating a heat transfer medium; a cladding pipe surrounding the metal pipe for forming an annular space that can be evacuated; a wall extending between the cladding pipe and the metal pipe for sealing the annular space; and a retaining device for a getter material or a container filled with getter material and/or protective gas, having a receiving section for receiving the getter material or the container. In addition, the invention relates to an absorber pipe, in particular for solar collectors in solar thermal power plants, having at least one collector mirror, comprising a metal pipe for conducting and heating a heat transfer medium; a cladding pipe surrounding the metal pipe for forming an annular space that can be evacuated; and a getter material disposed in the annular space for binding free hydrogen present in the annular space.
2. Description of Related Art
Solar collectors, for example, can be equipped with a parabolic mirror, also called a collector mirror, and are used in so-called parabolic trough power plants. In known parabolic trough power plants, for example, a thermal oil that can be heated up to approximately 400° C. by means of solar rays reflected from the parabolic mirrors and focused onto the absorber pipe is used as the heat transfer medium. The absorber pipe is thus usually composed of a metal pipe, which has a radiation-absorbing layer and a cladding pipe typically made of glass, which surrounds the metal tube. The heated heat transfer medium is conducted through the metal pipe and, for example, is introduced into a device for producing steam, by which the heat energy is converted into electrical energy in a thermal process. The metal pipe and the cladding pipe run parallel and concentrically to one another. An annular space, which is sealed axially by a wall that is usually composed of metal, is formed between the metal pipe and the cladding pipe. The individual absorber pipes are welded together approximately at 4 m or longer lengths and are formed into solar field loops with a total length of up to 800 m. Absorber pipes of this type are known, for example, from DE 102 31 467 B4.
Commonly used heat transfer media, and thermal oils in particular, with increasing aging, release hydrogen, which is dissolved, for example, in the thermal oil. The quantity of dissolved hydrogen depends on the thermal oil used and on the operating conditions of the oil circuit.
The decomposition rate and thus the formation rate of hydrogen increases with increasing temperature. The decomposition of the thermal oil can be accelerated additionally by contaminants, for example by water, which gains access to the oil circuit by leakages in the heat exchanger. As a consequence of permeation through the metal pipe, the hydrogen being released gains access to the evacuated annular space, the permeation rate through the metal pipe also increasing with increasing operating temperature of the metal pipe. As a consequence of this, the pressure in the annular space also increases, which has as a consequence an increase in heat conduction through the annular space, which in turn leads to heat losses and to a lower efficiency of the absorber pipe or the solar collector. As a final effect, the service life of the absorber pipe is reduced, since after a certain time, a sufficient heat output can no longer be generated in order to be able to effectively conduct the thermal process.
In order to at least reduce the pressure increase in the annular space and thus to prolong the service life of the absorber pipe, the free hydrogen that has entered the annular space can be bound by getter materials. The absorption capacity of the getter materials is limited, however. After reaching the maximum loading capacity or after saturation of the getter material, the pressure increases in the total annular gap, dependent on the hydrogen partial pressure of the gas phase, until it is in equilibrium with the partial pressure of the free hydrogen that has dissolved out of the thermal oil. Previously, equilibrium pressures of several millibars (mbars) could be detected by means of field measurements. Due to the free hydrogen, increased heat conduction arises in the annular gap with the above-named disadvantageous consequences for the efficiency of the solar collector.
Absorber pipes, which are provided with getter materials in the annular space, are known, for example, from WO 2004/063640 A1. A retaining device for getter material, in which the getter material is stored in a trough-shaped track or loop, is described herein. The loop is attached via feet to the metal pipe. The feet are welded to the metal pipe, so that leakage can readily occur here, whereupon the heat transfer medium can enter the annular space and the vacuum in the annular space can be lost. In addition, it is a disadvantage in this retaining device that the strong temperature differences occurring during operation between the metal pipe and the carrier device and thus different length expansions must be considered, in order to prevent a buckling or a tearing off of the loop, which requires an increased expenditure for construction.
Further, the loop is found in a region that can be subject to direct solar radiation. In particular, rays that come from the mirror and miss or only brush against the metal pipe (defocused radiation) can lead to a heating of the loop and thus of the getter material. This is disadvantageous because the absorption capacity of the getter material for free hydrogen decreases with increasing temperature of the getter material, so that hydrogen that is already bound to the getter material is again released, whereby the pressure in the annular space and thus the heat conduction through the annular space again increase. Since the loop is joined via the feet directly to the metal pipe, a heat transfer, in particular a conductive heat transport, to the getter material occurs over it, which contributes to its heating.
As already mentioned initially, absorber pipes of this kind usually have walls with which the annular space is sealed. For this purpose, they extend between the metal pipe and the cladding pipe. Since the metal pipe and the cladding pipe are composed of different materials and are heated very differently during operation of the absorber pipe, they expand very differently, particularly in the axial direction. The wall comprises an expansion-equilibrating unit, with which the different thermal expansions can be equilibrated. Expansion-equilibrating units are manufactured at least partially of metal, so that they are impermeable to solar radiation. Consequently, the heat transfer medium in the region that is surrounded by expansion-equilibrating units is not heated, so that the efficiency of the absorber pipe deteriorates, the larger the region surrounded by expansion-equilibrating units.
On the other hand, the getter material can be advantageously disposed in the expansion-equilibrating units. Since, as described above, they are impermeable to solar radiation, the solar rays cannot reach the getter material or at least reach it only to a reduced extent and correspondingly do not heat it or heat it less strongly. Consequently, the absorption capacity of the getter material for free hydrogen is not reduced by solar radiation or at least is reduced less strongly in comparison to direct irradiation. A corresponding arrangement of the getter material is known from DE 10 2005 022 183 B3.
In order to increase the efficiency of the absorber pipe, however, one attempts to design expansion-equilibrating units as small as possible, in order to minimize the region of the absorber pipe surrounded by them. In this connection, one speaks of an enlargement of the aperture area of the absorber pipe, whereby the aperture area denotes the region of the absorber pipe that is accessible in an unhindered manner to solar radiation. Together with minimizing the region that is surrounded by expansion-equilibrating units, the space that is available for arranging the getter material in the expansion-equilibrating units is also minimized. Thus, a situation may occur, in which sufficient getter material can no longer be disposed in the expansion-equilibrating units, so that the quantity of hydrogen released during operation of the absorber pipe can no longer be adsorbed to the required extent. The absorption capacity for free hydrogen is proportional to the quantity of getter material utilized. Consequently, in the case of absorber pipes with maximized aperture area, the absorption capacity of the getter material is exhausted prematurely and the efficiency of the absorber pipe decreases prematurely, so that it needs to be changed prematurely for a new absorber pipe, which negatively influences the economic balance.
Absorber pipes currently available on the market are provided with an expansion-equilibrating unit, which either extends into the annular space between the absorber pipe and the cladding pipe (DE 102 31 467 B4) or which joins the absorber pipe and the cladding pipe on the outside with one another (DE 60 223711 T2). With a temperature increase of the absorber pipe, the expansion-equilibrating unit extending into the annular space is thus compressed, whereby the aperture of the absorber pipe increases under the temperature conditions during operation.
The wall with which the annular space is sealed is composed of metal, at least in sections, so that a glass-metal connection must be provided at the end of the cladding pipe. Since metal and glass directly transition into one another in the glass-metal connection, the different length expansions due to a temperature change are particularly critical here. As a consequence of the different length expansion, damage occurs frequently at the glass-metal connection, which leads to a loss of the vacuum in the annular space. This results in a clear reduction in the efficiency of the solar collector, which then can no longer be operated economically.
The expansion-equilibrating unit extending toward the annular space screens the half of the glass-metal connection turned away from the collector from defocused, concentrated radiation. The compression of the expansion-equilibrating unit that accompanies higher temperatures can lead to the circumstance that the glass-metal connection is subjected to defocused radiation, particularly in the case of an axially shortened configuration of the expansion-equilibrating unit.
In the case of the outer-lying expansion-equilibrating unit, the latter offers no protection for the glass-metal connection. Therefore, a shield is provided elsewhere for the protection of the glass-metal connection (DE 60 223 711 T2).
The defocused radiation contributes to the heating of the glass-metal connection, but not to the heating of the thermal oil, so that it provides no contribution to the generation of electrical energy. Thus, the efficiency of the solar collector decreases with an increasing fraction of defocused radiation. Secondary mirrors, which are disposed in the annular space in the half of the absorber pipe turned away from the collector mirror in order to increase the efficiency of the solar collector, are known from U.S. Pat. No. 4,432,345 and U.S. Pat. No. 4,273,104.
The problem of the present invention is thus to at least reduce the above-discussed disadvantages of known retaining devices of the prior art and to further develop the absorber pipe so that the heating of the getter material is at least reduced and a simple manufacture and assembly of the absorber pipe is made possible, whereby the retaining device can be supplied with both getter material as well as with a container that is filled with getter material and/or protective gas, and the getter material will be arranged as desired.
In addition, the problem of the present invention is to respond to the disadvantages of known absorber pipe designs, in particular the reduction in the capacity of the getter materials for free hydrogen and the heating of the glass-metal connection due to defocused radiation and the thus accompanying loss of defocused radiation.